The present invention relates to a digital signal reproducing apparatus suitable for reproducing digital PCM audio signals that have been recorded in the form of single helical tracks on a recording medium, one track being formed per unit time, with a rotary head.
A technique is known in which audio signals are recorded on magnetic tape with a helical scanning rotary head in the form of helical tracks, one track being formed per unit time, and reproduced thereafter. A digital signal record/reproduce apparatus known as R-DAT (rotary head type digital audio tape recorder) has been designated for recording audio signals as PCM signals and thereafter reproducing the same.
A format of tracks to be recorded in the actual system of R-DAT has a pattern as shown in FIG. 4(a), in which each of MARGIN, PLL and POSTAMBLE has a frequency of 1/2 f.sub.M (f.sub.M =9.4 MHz) and IBG a frequency of 1/6 f.sub.M. Each of SUB and PCM is composed of a plurality of blocks as shown in FIG. 4(b). SYNC is composed of 10 bits in which 9 bits are fixed with the remainder assuming various patterns depending upon the place and audio signals. SUB consists of a cyclic pattern of 8 such blocks, and PCM 128 blocks. The numerals given in FIG. 4(a) represent the numbers of blocks occupied by the respective regions. ATF-1 between SUB-1 and PCM and ATF-2 between PCM and SUB-2 are each a region (ATF=automatic track finding) provided for ensuring that tracking control, i.e., control for allowing a rotary head to correctly scan the recorded tracks during reproduction, can be accomplished by means of the output of the head without employing any special head.
In R-DAT, PCM signals compressed on a time base are recorded in the form of helical tracks on magnetic tape by means of two rotary heads. Instead of providing a guard band between adjacent tracks, a tracking pilot signal is recorded both at the beginning and at the end of each track in a region independent of the area in which the PCM signals are recorded. During reproduction, the recorded tracks are scanned with a rotary head having a scanning width larger than the width of each track, and the reproduction output of the pilot signals from the two tracks adjacent to the track being scanned is used to control the tracking of the rotary head.
The track pattern for ATF is specified as shown in FIG. 5 and is hereinafter described with reference to the case where a drum having diameter of 30 mm is rotating at 2,000 rpm with the tape wound at an angle of 90.degree. to the drum.
ATF-1 and ATF-2 located in the front and rear portions, respectively, of each track have a low-frequency (small azimuth-effect) signal f.sub.1 as a tracking pilot signal. This signal is used for the purpose of detecting the levels of crosstalk resulting from the two tracks adjacent to the track being reproduced, so as to obtain the difference between the levels of such crosstalk as a tracking error signal. A low-frequency signal of f.sub.M /72 (130 kHz) is used as the pilot signal f.sub.1.
In each Of ATF-1 and ATF-2 is recorded a sync signal for identifying the location at which the pilot signal f.sub.1 is recorded. In the presence of crosstalk, the sync signal is unable to distinguish the on-track from adjacent tracks, so it is selected in such a way that it not only has a frequency capable of producing an azimutheffect but also affords a pattern that is not possessed by the PCM signal. If the head having a + (plus) azimuth is designated A and the head having a -(minus) azimuth is designated B, two different sync signals are provided for the purpose of distinguishing head A from head B. Stated more specifically, a sync 1 signal f.sub.2 having a frequency of f.sub.M /18 (=522 kHz) and a sync 2 signal f.sub.3 having a frequency of f.sub.M /12 (=784 kHz), associated with heads A and B, respectively, are recorded in predetermined positions.
In R-DAT which does not employ an erase head, a new signal is written over the previously recorded signal. In order to enable this "overwrite" mode, an erase signal f.sub.4 having a frequency of f.sub.M /6 (=1.56 MHz) is recorded at a predetermined position for erasing the previously recorded pilot signal f.sub.1, sync 1 signal f.sub.2, and sync 2 signal f.sub.2.
The pilot signals for ATF are located at different positions on the on-track and the two adjacent tracks and the level of the pilot signal on the on-track (i.e., the track being scanned) differs on a time basis from the level of each of the pilot signals on the adjacent tracks, so that the three different levels can be samples independently of each other.
Five blocks are assigned to each of the ATF regions, ATF-1 and ATF-2, and the pilot signal f.sub.1 is recorded in two of these blocks. The sync signal f.sub.2 is recorded in an area covering 1 or 0.5 block from the center of the position in which one of the two adjacent tracks is recorded. The pilot signal f.sub.1 on the other adjacent track is recorded in such a way that its center is positioned two blocks after the beginning of the sync signal recorded on the on-track. A sync signal composed of one block is assigned to an odd-number frame, and a sync signal composed of 0.5 blocks is assigned to an even-number frame.
As described above, the sync signals to be recorded in the ATF region have different frequencies depending upon which head is used in scanning, and these sync signals also have different recording lengths in odd-number frames and even-number frames. This design enables four consecutive tracks to be distinguished from one another since they are provided with different ATF regions. The pattern of ATF regions is of a 4-track completed type in which it is cyclically repeated for every 4 tracks.
When magnetic tape in which signals have been recorded in the format shown in FIG. 4(a) is played back with a rotary head, an RF signal of the type shown in FIG. 6(a) is reproduced from the head. If this RF signal is obtained by playback of a track with the odd-number frame (A) shown in FIG. 5, it may be passed through a bandpass filter (BPF) of 130 kHz to obtain a pilot signal f.sub.1 as shown in FIG. 6(b).
The signal in zone I is due to the pilot signal on the on-track, and those in zones II and III result from the crosstalk of the pilot signal on a track with the odd-number frame (B) and a track with the even-number frame (B), respectively. If the rotary head were scanning the on-track correctly, the envelope levels of zones II and III, or the values of V.sub.II and V.sub.III indicated in FIG. 6(c) should be equal to each other. However, if a tracking deviation occurs, V.sub.II is not equal to V.sub.III (V.sub.II .noteq.V.sub.III) and the amount and direction of the deviation of the rotary head with respect to the on-track can be determined by the magnitude and polarity of the difference between V.sub.II and V.sub.III. Therefore, by actuating a capstan servo according to the difference between V.sub.II and V.sub.III to effect fine adjustment of the tape speed, the rotary head can be controlled to travel correctly on the on-track. To this end, it is necessary to correctly detect a certain sync signal at a predetermined position and to form windows for achieving correct detection of various signals that are recorded at predetermined positions.
In order to form windows for various recorded signals in R-DAT, a data sync is detected and a system counter for window formation is operated with the detected data sync used as a reference. However, the detection of data sync alone is not sufficient for the purpose of correctly detecting the running position of the tape. For instance, a dropout sometimes renders it impossible to correctly detect the subcode or PCM data sync from the first track, and erroneous detection of the data sync can occur if there is a clock mismatch or if a slight offset occurs in the threshold point. Furthermore, the slightest fluctuation in the RF signal can also cause erroneous detection of the data sync. Therefore, if the data sync is to be used as a reference for the operation of the system counter, the width of the window used to achieve error-free detection of various recorded signals must inevitably be increased and it becomes difficult to form windows having an appropriate width.
Another reason for the necessity to employ wide windows in the prior art R-DAT is that the width of data sync windows is determined solely on the basis of a reference clock produced from a crystal oscillator. As a further problem, the reliability of the system counter operating for window formation during data reproduction is largely dependent on the conditions of the tape running.
The data sync is inherently prone to erroneous detection on account of such factors as clock mismatching and the occurrence of small dropouts and noise, and the chance of erroneous detection is increased if wide windows are formed. Potential offsetting due to failure to closely follow the data must also be taken into account. The inherent nature of a sync window is such that if it is too narrow, the sync signal to be detected may be missed and that if it is too broad, erroneous detection can occur.
A PLL clock produced by extracting a clock component from a data train is capable of closely following the data, but in R-DAT, data recording or reproduction alternates with signals that have a different character from the data (e.g., signals whose frequency is completely fixed). Therefore, comparatively wide sync windows must be formed so that they can be used as a reference for data reading but then, this reduces the operational precision.
In a replay mode of R-DAT, the data region recorded on each track in accordance with a predetermined format and the non-data region are successively reproduced. During reproduction of the data region, a PLL system counter operates in response to a clock that is extracted from a data train by a PLL circuit and which is capable of closely following the data, and the operation of this PLL system counter starts at the time when the data sync that serves as a reference for reading the data recorded on the predetermined format has been detected. During reproduction of the non-data region, a crystal system counter is operated in response to a clock from a crystal oscillator. In this way, the PLL system counter and the crystal system counter detect the actual positions at which the data region and the non-data region are respectively reproduced from the tape. However, when the mode of reproduction is switched from the non-data region to the data region, both system counters will stop operating for a certain period of time. It has been difficult to form a windowing this period that has an appropriate width for correctly detecting and protecting the data sync or the tracking ATF sync.
The closer the width of the window is to the duration of a signal of interest, the more immune the system is to erroneous detection that may be caused on account of extraneous factors such as noise, and the better the S/N ratio that can be ,attained on a time base. However, if the duration of the window is too strictly determined in consideration of jitter and other factors, a disadvantage will occur in that the window with such a small tolerance on a time basis prevents the reading of data that could theoretically be read. Furthermore, as shown in FIG. 4(a), R-DAT employs a track format consisting of a sequence of MARGIN, PLL, SUB-1, POST AMBLE, IBG, ATF-1, IBG, PLL and PCM. Since MARGIN and PLL have a frequency of 1/2 f.sub.M, they can theoretically be distinguished from the data sync for SUB-1 and correct data detection should be possible. In fact, however, the data becomes indistinguishable from the data sync on account of jitter, dropouts and other extraneous factors, and if a certain data sync is erroneously detected, the error may affect one block of data. In order to avoid this problem, the data must be protected with an appropriately set window.
As for the ATF region, the IBG is present on both sides and the two regions can theoretically be distinguished from each other since IBG has a frequency of 1/6 f.sub.M. But in this case, too, some signals may be erroneously identified as SY2 (sync Signal for head A) or as SY3 (sync signal for head B) and there is at least the possibility that SP1, a sampling signal for actuating the ATF operation, will be generated. Furthermore, if erroneous detection occurs after "overwriting", SP2 may be generated two blocks after the generation of SP1. It is therefore necessary to protect the AFT by setting an appropriate window.
For the reasons described above, the sync window should be set in such a way that its width is the closest possible to the duration of a signal of interest, but the data that can be used to set a window will vary with the data that has been read after head switching. Ideally, data should be read in a prescribed order but if a dropout should occur in the subcode area to make it impossible to read data in SUB-1, it is desired to read data in ATF-1 and use it as a reference signal for window setting; if data cannot be read in SUB-1 or ATF-1, data is desirably read in PCM and used as a reference signal for window setting. If the failure to read data occurs in other regions, there is no need to take special provisions because the data read in by such provisions will not make any sense in almost all situations.
The R-DAT described above is designed so that a certain time exists between head switching and the actual reproduction of an RF signal. A pulse generator (PG) generates a head switching pulse (PG pulse) as the rotary drum that is equipped with two heads spaced at an angular distance of 180 rotates on the track format to achieve data reproduction in the order of SUB-1, ATF-1, PCM, ATF-2, and SUB-2. With a PG pulse generated in such manner being used as a reference, a sync window is set that it will have a width so that is equal to the duration of time over which a certain signal to be reproduced would actually exist.
However, in this case, too, the interval between the generation of a PG pulse and the actual production of an RF signal may be unexpectedly long or may fluctuate in the presence of jitter, and if a dropout occurs, a desired signal may be absent from the position where it should appear. Under these circumstances, the very fact that a sync window exists prevents reading of data that could theoretically be read.